Your Bitbucket Server user tier does not need to match your JIRA licensing tier in order to integrate the two products. Only users who are given the 'Bitbucket Server User' permission, through either the JIRA user management or internal Bitbucket Server user management, will count against your Bitbucket Server license. 

If you plan to use JIRA for user management, then you'll have the ability to create a group within JIRA to contain all of your Bitbucket Server users and give only those users the right to log into Bitbucket Server. I suggest reviewing Delegate user management to Jira for more information.